All fiber autocorrelator

ABSTRACT

An autocorrelator apparatus and method for economically measuring physical properties of an object where the measurement path is at least semi-translucent to light, such as thicknesses in multilayered optical structures, group index of refraction, and distance to a surface. The apparatus includes a non-coherent light fiber interferometer and an optional coherent light fiber interferometer in association so as to share PZT fiber modulators. Thickness and boundary extent measurements can be made, for example, of solids, liquids, liquids moving along a horizontal plane, or liquids flowing down a plane.

FIELD OF INVENTION

[0001] The invention relates generally to autocorrelators constructedfrom optical fiber instead of bulk optics useful for measuringreflection areas in materials and physical properties thereof that areat least semi-transparent to light.

BACKGROUND OF THE INVENTION

[0002] In web and coating manufacturing operations, expensive bulk opticinterferometric apparatus are used for accurate, on-line measurements ofweb and coating layer thickness. Apparatus, such as shown in U.S. Pat.No. 5,633,712, by Venkatesh, et al., U.S. Pat. No. 5,659,392, by Marcus,et al. which issued Aug. 19, 1997 and the associated method taught inU.S. Pat. No. 5,596,409 by Marcus, et al. which issued Jan. 21, 1997have a high degree of lateral resolution, are light weight, compact,easy to set up, and are robust in high and low temperature environments,in the presence of solvents, high air flow, and various levels ofrelative humidity. Such apparatus are self-calibrating or able to remainin calibration for extended periods of time so that the apparatus can beinstalled on a production machine without the need for re-calibration.Unfortunately, the expensive bulk optics and mechanics required in suchdevices reduce their usefulness except in high value productionapplications. In addition, the mechanical nature of the scanning opticsof such devices reduce the possible scan rate and service life.

[0003] Therefore, there has been a need for an economical, long lifetimemeasuring device, which can produce at least as accurate measurements athigher scan rates that requires little periodic maintenance and can bepackaged with minimal size and mass.

SUMMARY OF THE INVENTION

[0004] In the present invention, the disadvantages of the prior artinterferometric devices are overcome by eliminating bulk optic andmechanical components, instead providing an all fiber device. Themechanical components and open optics are eliminated and instead,scanning is accomplished by means of piezo electric fiber stretchersusually configured in tandem to proved a rapid, accurate measurementwith a relatively large dynamic range. Replaceable sensing probes may beany length and to do not require length matching to other components ofthe measuring instrument.

[0005] In the present invention, broadband light (of a first wavelength)is guided by a probe fiber of arbitrary length to the sample material.Multiple reflections of the broadband light from the sample areoppositely guided back through the probe fiber, and subsequently guidedto an all-fiber, optical path matched autocorrelator assembly where thislight is split into two beams which each pass through fiber stretchers,which are driven in opposite directions. Both beams are reflected backupon themselves so as to double pass their respective fiber stretchersto be recombined with the original splitter. The recombined signal isguided to a photodetection device creating a first electronic signalrepresentative of the scanned reflections from the sample material.

[0006] The present invention also allows for a wavelength stablecoherent optical source of a second wavelength different than that ofthe broadband source to be co-injected to the autocorrelator assemblywith the broadband light. This second wavelength co-propagates with thefirst wavelength through the autocorrelator taking the same paths of thefirst wavelength and is also recombined with the original splittercausing an interferometric fringe rate proportional to the optical pathvariations caused by the fiber stretchers. This second wavelength isthen guided to a separate photodetection device causing a secondelectronic signal which represents a highly accurate measure of theautocorrelator scan. This second electronic signal may be used to assistin the interpretations of the first electronic signal such that a highlyaccurate displacement measure of the multiple reflections from thesample material may be made. These interpretations in turn may be usedto accurately define physical properties of the sample material.

[0007] Therefore, it is an object of the present invention to provide animproved white light interferometric reflective measuring device at afraction of the cost of similar measuring devices available in the priorart.

[0008] Another object is to provide a white light interferometricreflective measuring device that implements its scanning mechanism bymeans of fiber stretching.

[0009] Another object is to provide a white light interferometricreflective measuring device that incorporates an optical path matchedautocorrelator section which is independent of the probe fiber, henceremoving the requirement of path matching for the probe fiber.

[0010] Another object is to provide a white light interferometricreflective measuring device with high resolution and fast scan rates.

[0011] Another object is to provide a white light interferometricreflective measuring device that provides for a first broad bandwavelength to probe the sample material and a second coherent wavelengthto measure the scan distance variations of the autocorrelator.

[0012] Another object is to provide a white light interferometricreflective measuring device that utilizes low cost single mode fiber andassociated low-cost single mode fiber components and implementsorthoconjugate mirrors (known as Faraday rotator mirrors (FRM)) in theautocorrelator section which assures both high interferometricvisibility and minimal birefringence modulation of the broadband light.This in turn provides the highest possible resolution capability of theinstrument.

[0013] These and other objects and advantages of the present inventionwill become apparent to those skilled in the art after considering thefollowing detailed specification, together with the accompanyingdrawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic diagram of a partial fiber configuredinterferometric reflective measuring device (Michelson white lightinterferometer) constructed in accordance with the prior art and havinga fiber probe;

[0015]FIG. 2 is a schematic diagram of a modified version of the deviceof FIG. 1 with an all fiber scanning assembly;

[0016]FIG. 3 is a schematic diagram of a all fiber autocorrelatorconstructed in accordance with the present invention;

[0017]FIG. 3A is a graph of output level vs, displacement for theautocorrelator of FIG. 3 when its probe is pointed at a rear surfacemirror; and

[0018]FIG. 4 is a schematic diagram of an enhanced version of theautocorrelator of FIG. 3 including a reference laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Referring to the drawings more particularly by reference numbers,number 10 in FIG. 1 refers to a prior art Michelson scanninginterferometer in which a broadband light source 12 provides white light13 through a fiber 14 to a polarizer 16. The polarizer 16 and thepolarization maintaining fibers 20, 22, 24, and 26 downstream therefromas well as a polarization maintaining fiber coupler are included toeliminate polarization fading.

[0020] The polarized white light 30 passes through the fiber 20 and issplit into two light beams 32 and 34 by the polarization maintainingcoupler 28. The beam 32 is projected out of a non-reflective fibertermination 36, through a focusing lens 38 and onto a mirror device 40.The mirror device 40 may be a corner cube or retro-reflector to assurethat the reflected reference beam 44 returns to the termination 36without much attenuation. The referenced beam 44 is formed bytranslating the device 40 typically by means of either a motorizedlinear slide, rocker assembly or beam deformer 45, whose motion is shownsymbolically by the arrow 46. Such techniques have proven to beeffective in providing appropriate scan ranges, but have a number ofundesirable features when being considered for instrument production.These include: high cost of the launch and collection optics, andspecialized motor controllers; low speed scan rates because inertialimits scan rate, and rocker assemblies limit range through anglechanges; the requirement for periodic maintenance such as opticalalignment and cleaning; the limited lifetimes inherent in mechanicalsystems with moving parts; and package limitations because compactpackages are delicate, so size and mass must be increased forrobustness.

[0021] The sensing beam 34 in the sensing leg of fiber 24 is projectedout of a fiber termination 50, through a focusing lens 52 and onto asample 54 whose spacing of reflective interfaces are to be determined.The fiber termination 50 may be a partial reflective termination toenable a reference distance to the sample by producing a referencereflection or the sensing leg fiber termination 50 may be non-reflectiveand a reference reflection be incorporated into the sample 54.

[0022] The beams 44 and 56 are combined in the polarization maintainingcoupler 28 as an optical signal 60 whose intensity varies with time inreference to the motion of the mirror device 40 where there will bezones of incoherent intensity summation and zones of coherentrecombination (of the two beams 44 and 56). The beam 60 passes throughfiber 26 to be projected onto an optical receiver 62 by a non-reflectivefiber termination 64. The optical receiver 62 converts the opticalintensity levels into electrical signals, which are digitized forsubsequent signal processing. Also digitized is the electrical monitoror pick-off signal along line 66 from the translating device 45 by thedata acquisition electronics 68.

[0023] The concept of an all-fiber Michelson white light interferometer70, as shown in FIG. 2, is highly appealing in that the light is selfcontained and eliminates the packaging complexities associated withintegration of bulk optics with fiber waveguides. A polarizationmaintaining fiber arrangement is shown. The scanner apparatus 70includes a broadband light source 72 that provides white light 73through a fiber 74 to a polarizer 76. The polarizer 76 and thepolarization maintaining fibers 80, 82, 84, and 86 downstream therefromas well as a polarization maintaining 50/50 fiber coupler 88 areincluded to eliminate polarization fading.

[0024] The polarized white light 90 passes through the fiber 80 and issplit into two light beams 92 and 104 by the coupler 88. The beam 92 ispassed through a piezoelectric fiber stretcher 95 and is reversed indirection by mirror 100 as reference beam 102.

[0025] In piezo-electric fiber stretchers 95, 105, typically a length offiber is wound around the circumference of a ceramic piezo cylinderelement with a sufficient tension that assures that the fiber never goeslimp. Using the white light interferometer configuration 70, withavailable fibers of reasonable length and appropriate piezo ceramicmaterial for the modulators, 10 mm of scan range can be obtained for lowfrequency scan rates and at 1 mm scans, an order of magnitude fasterscan rate can be produced. It is typical that when using 2.3 inchdiameter cylinders, each with 40 meters of fiber applied that 10 mmscans at 50 Hz rates and 1 mm scans at 500 Hz may be achieved.

[0026] The sensing beam 104 in the sensing leg of fiber 84 is passedthrough a piezoelectric fiber stretcher 105, driven opposite to thestretcher 95, and is projected out of a fiber termination 110, through aoptional focusing lens 112 which forms the probe 113, and onto a sample114 whose spacing of reflective interfaces are to be determined. Likebefore, the fiber termination 110 may be a partial reflectivetermination to enable a reference distance to the sample 114 byproducing a reference reflection in the return reference beam 116 or thesensing leg fiber termination 110 may be non-reflective and a referencereflection be incorporated into the sample 114.

[0027] The beams 102 and 116 are combined in the coupler 88 as aninterference beam 120 whose intensity varies with time in reference tothe stretching of the fibers 82 and 84. To assure that interferencebetween the two beams 102 and 116 occurs, the pathlengths out and backto the coupler 88 must be very close, since any mismatch reduces thedynamic measuring range of the instrument 70.

[0028] The beam 120 passes through fiber 86 to be projected onto anoptical receiver 122 by a non-reflective fiber termination 124. Theoptical receiver 122 converts the intensity changes in the beam 120 intoelectrical signals, which are demodulated in a demodulator 124 inaccordance to the stretching by the piezoelectric fiber stretchers 95and 105.

[0029] The PM fiber arrangement in FIG. 2 is the design of choice. Theuse of lower cost single mode fiber scanners with PZT modulatorsproduces birefringence modulation caused by the modulation process,which broadens the coherence which reduces measurement resolution andalso causes polarization fading although such are available for very lowcost applications.

[0030] A variation of the All-Fiber Michelson white light interferometer70 is realized when the probe 113 is located external to theinterferometer 70. In this case, the light returning from a sample issent to a scanning interferometer and then processed. This all-fiberautocorrelator 150 as shown in FIG. 3, has the advantage of using aprobe of arbitrary length without having to match the length of a probefiber to the length of a reference fiber. Another advantage involves theuse of lower cost single mode fiber to replace the polarizationmaintaining fiber where no birefringence modulation degradation isexperienced when Faraday rotator mirrors are used.

[0031] The all-fiber autocorrelator 150 is shown in FIG. 3 in its mostgeneric form. Here the same type of fiber modulator scanning mechanismas in instrument 70 is used. A disadvantage to this approach whencompared to the Michelson approach is that it has a larger optical lossresulting from the extra coupler. This coupler could however be replaced(at an additional expense) with a circulator to improve the throughputpower such that is roughly equivalent to that of the Michelson approach.Use of the circulator also provides immunity of the broadband sourcefrom back reflected light from the sample.

[0032] Unlike before, a single mode fiber arrangement is shown. Theautocorrelator 150 includes a broadband light source 152 that provideswhite light 153 through a single mode fiber 154. The single mode fiber154 and the single mode fibers 161, 162, 163, 164 and 165 downstreamtherefrom as well as 50/50 single mode fiber couplers 166 and 168 formthe primary light paths for the autocorrelator 150.

[0033] The white light 153 passes through the fiber 154, coupler (orthree port circulator) 166, fiber 161 and out of a probe 171 forreflection off the sample 172 under test. The reflected beam 174 isre-acquired by the probe 171 is conducted by the coupler 166 and fiber162 to the second 50/50 single mode coupler 168 where it is split intotwo light beams 175 and 176. The probe 171 could include a separateoptical fiber for re-acquiring the reflected beam 174, in which case,coupler 166 can be eliminated. The beam 175 is passed through a singlemode fiber wound piezoelectric fiber stretcher 177 and is reversed indirection by Faraday rotator mirror 180 providing an orthoconjugatereflection causing a 908 polarization rotation as first reference/signalbeam 182. The beam 176 is passed through a second piezoelectric fiberstretcher 184 driven opposite from stretcher 177 and is reversed indirection by Faraday rotator mirror 185 where its polarization is alsorotated by 90° as second reference/signal beam 186. The second fiberstretcher does not need to be present when a reduced measurement rangeis all that is required so long as the light beam 176 travels a similarlight path distance to that of light beam 175. Passage through thestretchers 177 and 184 can cause birefringence variations so shiftingthe return beams state of polarization by 90° causes any birefringencevariations of the light going through in one direction to be correctedduring the reverse passage. The piezo-electric fiber stretchers 177 and184 are constructed as described for stretchers 95 and 105, with thefibers 163 and 164 forming the light guides thereof being optical pathmatched.

[0034] The first and second reference/signal beams 182 and 186 arecombined into an interference beam 190 by the single mode coupler 168and conducted by single mode fiber 165 to receiver and demodulatorelectronics 192. The response of the autocorrelator 150 shown as arectified envelope of the inteferogram from a single autocorrelator scanof a rear surface mirror having a partial reflecting front surfaceseparated by a distance X to the rear reflector as the sample is shownin FIG. 3A with the probe 171 having no reflection. The first pulse 195represents interference between the front and rear surfaces (atdisplacement −X). The second pulse 196 represents the interferencebetween the front surface with itself summed with the interference ofthe rear surface with itself (at zero displacement). The third pulse 197represents the interference between the front and rear surface (atdisplacement +X).

[0035] The autocorrelator 250 shown in FIG. 4 uses an additionalcoherent optical source which co-propagates with the broadband lightinside the scanning interferometer. Wavelength division multiplexer's(WDM) or other appropriate combining/splitting and filter elements areused to inject and separate out the returns from the broadband andcoherent sources. The detected fringe crossings from the coherent sourceare used to determine the exact displacement of the scan at all pointsin the sweep.

[0036] The modified autocorrelator 250 is shown in FIG. 4 is essentiallyidentical to the autocorrelator 150 except for modifications to allowinjection and separation of returns from a coherent source so thatdetected fringe crossings from the coherent source can be used todetermine the exact displacement of the scan at all points in the sweep.The modified autocorrelator 250 includes a broadband light source 252that provides white light 253 at a center frequency λ₁ through a fiber254 to a 50/50 coupler 256, or in applications that require low lightloss, a three port circulator. The coupler 256 is shown with atermination 257 on its unused leg 258. The coupler 256 and the singlemode fibers 260, 261, 262, 263, 264, 265, 312, and 266 downstreamtherefrom as well as 50/50 fiber coupler 268 form the main sensing lightpath of the autocorrelator 250.

[0037] The white light 253 passes through the fiber 260 and out of aprobe 270 for reflection off the sample 271 under test. Like before, theprobe 270 includes a fiber termination 272 and an optional focusing lens273. The reflected beam 274 is re-acquired by the probe 270, and isconducted by the coupler 256, fiber 261, and a wavelength divisionmultiplexer 275 (or coupler used to combine the two beams) to the second50/50 coupler 268 where it is split into two light beams 276 and 277.The beam 276 is passed through a piezoelectric fiber stretcher 278 andis reversed in direction by Faraday rotator mirror 280 where its stateof polarization is rotated 90° as first reference/signal beam 282. Thebeam 277 is passed through a second piezoelectric fiber stretcher 284driven opposite from stretcher 278 and is reversed in direction byFaraday rotator mirror 285 where its state of polarization is rotated90° as second reference/signal beam 286. The piezo-electric fiberstretchers 278 and 284 are constructed as described for stretchers 95and 105, with the fibers 263 and 264 forming the light guides thereofbeing optical path matched.

[0038] The first and second reference/signal beams 282 and 286 arecombined into an interference beam 290 on fiber 265 by the coupler 268and directed through a fiber termination 291 to an optical receiver 292by a WDM 293 or other appropriate splitter and filter.

[0039] The autocorrelator 250 shown in FIG. 4 uses a coherent opticalsource 296 and injects a coherent beam 298 into fiber 299 at a frequencyλ₂, that is not common with any of the frequencies centered at λ₁, bymeans of the WDM 275 positioned between the couplers 256 and 268. It isimportant that λ₂ be selected such that it can propagate single modethrough the same fiber and couplers used for λ₁ and also it be closeenough in wavelength to λ₁ (25% is sufficient) so that the couplers andFaraday rotator mirrors (which are typically adjusted to λ₁) are able to(but with small errors) function correctly for this second wavelength.For example, if standard telecommunications single mode fiber is used,selections of λ₁ and λ₂ of 1300 nm and 1550 nm satisfies the criteria.Commercial devices (wide band or dual window couplers, single modefiber, circulators, Faraday rotator mirrors, and WDM's) are abundant atthese two wavelengths. The coherent beam 298 co-propagates with thebroadband light inside the scanning interferometer 250. The coherentbeam 298 is split by the coupler 268 into coherent beams 300 and 302.The coherent beams 300 and 302 are passed through the stretchers 278 and284, reflected off the Faraday rotator mirrors 280 and 285, and passedagain though the stretchers 278 and 284 for combination on the coupler268 into coherent fiber modulator sensing beam 306. The beam 306 isconducted along fiber 265 and is separated by the WDM 293 onto a secondoptical receiver 310 by means of fiber 312 and termination 314. WDM's orother appropriate splitter/filters are used to inject and separate outthe returns from the broadband and coherent sources. The detected fringevariations from the coherent source 296 are used to determine the exactdisplacement of the scan at all points in the sweep.

[0040] Thus, there has been shown novel all-fiber autocorrelators whichfulfill all of the objects and advantages sought therefor. Many changes,alterations, modifications and other uses and applications of thesubject invention will become apparent to those skilled in the art afterconsidering the specification together with the accompanying drawings.All such changes, alterations and modifications which do not depart fromthe spirit and scope of the invention are deemed to be covered by theinvention which is limited only by the claims that follow.

1. An autocorrelator apparatus for measuring the distance from whichlight is reflected including: a broadband light source for producing afirst light beam; a fiber light probe connected to receive the firstlight beam and shine it on a structure that reflects at least a portionof the first light beam as a second light beam into said fiber lightprobe; a first optical fiber; a second optical fiber, said secondoptical fiber having the near identical light path length as said firstoptical fiber; first fiber means for splitting the second light beaminto third and fourth light beams into said first and second opticalfibers respectively; a first fiber stretcher to vary the optical pathlength of said first optical fiber; a first reflector positioned toreflect the third light beam back though said first optical fiber andsaid first fiber stretcher as a fifth light beam; a second reflectorpositioned to reflect the fourth light beam back though said secondoptical fiber as a sixth light beam, said first fiber means combiningthe fifth and sixth light beams into a seventh light beam; and a firstoptical receiver positioned to receive intensity variations in theseventh light beam and a signal representative of the variation in lightpath length of said first optical fiber and to produce therefromindications of the displacement of reflections of the first light beamfrom said fiber light probe.
 2. The autocorrelator apparatus as definedin claim 1 further including: a second fiber stretcher to vary theoptical path length of said second optical fiber opposite from thevariation of the optical path length of said first optical fiber.
 3. Theautocorrelator apparatus as defined in claim 1 wherein said first fibermeans include: a single mode fiber coupler, and wherein said first andsecond fibers are single mode fibers.
 4. The autocorrelator apparatus asdefined in claim 1 wherein said first fiber means include: a fibercoupler.
 5. The autocorrelator apparatus as defined in claim 1 whereinsaid first fiber means include: a single mode fiber coupler, whereinsaid first and second fibers are single mode fibers, and wherein saidfirst and second reflectors are Faraday rotator mirrors.
 6. Theautocorrelator apparatus as defined in claim 1 further including: acoherent light source for producing an eighth light beam of a wavelengthfrequency different from the first light beam; means to couple theeighth light beam into said first and second optical fibers so t hatsaid first and second reflectors reflect ninth and tenth light beamstherefrom which are combined at said first fiber means into an eleventhlight beam; a second optical receiver positioned to detect fringevariations of the eleventh light beam to determine the exactdisplacement of the scan of the first fiber stretcher at all points inits sweep.
 7. The autocorrelator apparatus as defined in claim 6 furtherincluding: a wavelength sensitive device connected to said first fibermeans to direct the seventh light beam to said first optical receiverand the eleventh light beam to said second optical receiver.
 8. Theautocorrelator apparatus as defined in claim 2 further including: acoherent light source for producing a eighth light beam of a wavelengthdifferent from the first light beam; means to couple the eighth lightbeam into said first and second optical fibers so that said first andsecond reflectors reflect ninth and tenth light beams therefrom whichare combined at said first fiber means into an eleventh light beam; asecond optical receiver positioned to detect fringe variations of theeleventh light beam to determine the exact displacement of the scan ofsaid first fiber stretcher at all points in its sweep.
 9. Theautocorrelator apparatus as defined in claim 8 further including: awavelength sensitive device connected to said first fiber means todirect the seventh light beam to said first optical receiver and theeleventh light beam to said second optical receiver.
 10. An apparatusfor measuring the distance from which light is reflected including: abroadband light source for producing a first light beam; a first fiberlight probe connected to receive the first light beam and shine it on astructure that reflects at least a portion of the first light beam as asecond light beam into said fiber light probe; a first fiber stretcherthrough which the second light beam is passed back and forth tocontrollably modulate the second light beam; and a first opticalreceiver positioned to receive intensity variations in the modulatedsecond light beam and a signal representative of the variation in lightpath length of said first optical fiber and to produce therefromindications of the displacement of reflections of the first light beamfrom said fiber light probe.
 11. The apparatus as defined in claim 10further including: is a second fiber stretcher through which the secondlight beam is passed back and forth to controllably modulate the secondlight beam opposite from the first light beam.
 12. The apparatus asdefined in claim 11 further including: a polarizer through which thefirst light beam is passed for establishing a polarization thereof; andpolarization preserving optical fiber through which the polarized firstlight beam and the second light beam are passed.
 13. The apparatus asdefined in claim 10 further including: a first Faraday rotator mirrorpositioned to reflect the second light beam back though said first fiberstretcher.
 14. The apparatus as defined in claim 11 further including:further including: a first Faraday reflector positioned to reflect thesecond light beam back though said first fiber stretcher; and a secondFaraday reflector positioned to reflect the second light beam backthough said second fiber stretcher.
 15. An autocorrelator apparatus formeasuring the distance from which light is reflected including: abroadband light source for producing a first light beam; a fiber lightprobe connected to receive the first light beam and shine it on astructure that reflects at least a portion of the first light beam as asecond light beam into said fiber light probe; a first optical fiber; asecond optical fiber, said second optical fiber having the nearidentical light path length as said first optical fiber; first fibercoupler for splitting the second light beam into third and fourth lightbeams into said first and second optical fibers respectively; a firstfiber stretcher to vary the optical path length of >said first opticalfiber; a first reflector positioned to reflect the third light beam backthough said first optical fiber and said first fiber stretcher as afifth light beam; a second fiber stretcher to vary the optical pathlength of said second optical fiber; a second reflector positioned toreflect the fourth light beam back though said second optical fiber andsaid second fiber stretcher as a sixth light beam, said first fibermeans combining the fifth and sixth light beams into a seventh lightbeam; and a first optical receiver positioned to receive intensityvariations in the seventh light beam and to produce therefromindications of the displacement of reflections of the first light beamfrom said fiber light probe.
 16. The autocorrelator apparatus as definedin claim 15 wherein said first fiber means include: a single mode fibercoupler, and wherein said first and second fibers are single modefibers.
 17. The autocorrelator apparatus as defined in claim 15 whereinsaid first fiber means include: a single mode fiber coupler, whereinsaid first and second fibers are single mode fibers, and wherein saidfirst and second reflectors are Faraday reflectors.
 18. Theautocorrelator apparatus as defined in claim 15 further including: acoherent light source for producing an eighth light beam of a wavelengthdifferent from the first light beam; means to couple the eighth lightbeam into said first and second optical fibers so that said first andsecond reflectors reflect ninth and tenth light beams therefrom whichare combined at said first fiber means into an eleventh light beam; asecond optical receiver positioned to detect fringe variations of theeleventh light beam to determine the exact displacement of the combinedscan of said first and second fiber stretchers at all points in theircombined sweep.
 19. The autocorrelator apparatus as defined in claim 18further including: a wavelength sensitive device connected to said firstfiber means to direct the seventh light beam to said first opticalreceiver and the eleventh light beam to said second optical receiver.20. The autocorrelator apparatus as defined in claim 17 furtherincluding: a coherent light source for producing an eighth light beam ofa wavelength different from the first light beam; means to couple theeighth light beam into said first and second optical fibers so that saidfirst and second reflectors reflect ninth and tenth light beamstherefrom which are combined at said first fiber means into an eleventhlight beam; a second optical receiver positioned to detect fringevariations of the eleventh light beam to determine the exactdisplacement of the combined scan of said first and second fiberstretchers at all points in their combined sweep.